DERBY-532 Support deferrable constraints

Patch derby-532-fix-drop-not-nullable. Fixes a broken predicate when
recreating the index when going from UNIQUE NOT NULL to plain UNIQUE:
the existing predicate missed the deferrable case, so the index was not
recreated.

Added a test case.



git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1550284 13f79535-47bb-0310-9956-ffa450edef68